if you just want to play those three games that you listed then a comp like mine is perfect but if you want the new games like doom3 and bf2 and halflife 2 then you are going to need to spend a lot more than 500. but if you are going to get one already made check out these comps at
www.ibuypower.com they have good gaming comps with just the computer no monitors or speakers or mouse keyboard etc. but they are good for gaming